I've been playing around for the past week with Tiny Twitter and I really do like it.   It's a Twitter client that allows you to keep up to date with your friends you are following as well as tweet directly from your device.   I used to tweet via SMS but now I've stopped doing that as well as cancelled all the updates via SMS and just rely on Tiny Twitter :)

The great thing is it's free and available for Windows Mobile devices (as well as a few other platforms) you can download it from